In the papermaking art, an aqueous suspension containing cellulosic fibers, and optional fillers and additives, referred to as stock, is fed into a headbox which ejects the stock onto a forming wire. Water is drained from the stock through the forming wire so that a wet web of paper is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine. Water obtained by dewatering the stock, referred to as white water, which usually contains fine particles, e.g. fine fibers, fillers and additives, is normally recirculated in the papermaking process. Drainage and retention aids are conventionally introduced into the stock in order to facilitate drainage and increase adsorption of fine particles onto the cellulosic fibers so that they are retained with the fibers on the wire. Cationic organic polymers like cationic starch and cationic acrylamide-based polymers are widely used as drainage and retention aids. These polymers can be used alone but more frequently they are used in combination with other polymers and/or with anionic microparticulate materials such as, for example, anionic inorganic particles like colloidal silica, colloidal aluminium-modified silica and bentonite.
U.S. Pat. Nos. 4,980,025; 5,368,833; 5,603,805; 5,607,552; and 5,858,174; as well as International Patent Application WO 97/18351 disclose the use of cationic and amphoteric acrylamide-based polymers and anionic inorganic particles as stock additives in papermaking. These additives are among the most efficient drainage and retention aids now in use. Similar systems are disclosed in European Patent Application No. 805,234.
It has, however, been observed that the performance of drainage and retention aids comprising cationic organic polymers deteriorates when used in stocks with high levels of salt, i.e. high conductivity, and dissolved and colloidal substances. Higher dosages of cationic polymer are normally required in such stocks but usually the drainage and retention effect obtained is still not entirely satisfactory. These problems are noticeable in paper mills where white water is extensively recirculated with the introduction of only low amounts of fresh water into the process, thereby further increasing the accumulation of salts and colloidal materials in the white water and the stock to be dewatered.